1. Field of the Invention
The present invention relates in general to an improved water-soluble solder flux and in particular to an improved water-soluble solder flux for rework or repair of circuit boards. Still more particularly the present invention relates to a water-soluble solder flux that does not require thermal activation and does not undergo significant chemical changes at repair temperatures and minimizes corrosive processes on a solder when utilized with the solder.
2. Description of the Related Art
In producing printed circuit boards the primary attachment and the rework/repair of components for these printed circuit boards have been accomplished utilizing rosin-based solder pastes and rework/repair fluxes. These organic, rosin systems require nonaqueous solvents, such as chlorofluorocarbons ("CFC"), for proper cleaning after the attachment or rework/repair of components.
Solder paste contains a flux as a component. Solder powder is suspended in the flux. This flux component of the solder paste is the material that controls many of the requirements of the solder paste. For example, the flux must remove metal oxides from the solder powder to allow reflow of all the solder properly.
The solder paste is screened onto a circuit board using an appropriately patterned stencil, in all of the locations requiring connection of components to copper pads. Next, components are placed on the circuit board on the prepared sites. The solder paste must be tacky enough to hold the components in their required location until the board is subject to reflow, at which time the solder paste flows to become the soldered connection for the components. Reflow is the term used to describe heating the solder above its melting temperature to give a soldered joint.
In producing printed circuit boards, testing of the boards is made, visually or through instruments, to find defects before sending the printed circuit board for shipment. Rework or repair includes any necessary repairs performed manually by operators after inspection of the completed circuit board.
Current solder pastes utilized in the industry have a flux system composed of a complex mixture derived from a natural product based pine-tar distillate fraction (called rosin). This rosin-based flux system requires an organic solvent to clean the flux residue after reflow, the heating of the solder above its melting temperature to give a soldered joint. The best solvent for cleaning is a halogenated hydrocarbon. With the current desire to decrease the use of halogenated hydrocarbons, this type of flux becomes less desirable.
Recently, in a global effort to curtail CFC emissions, water-soluble solder pastes and water-soluble rework/repair fluxes have been introduced which may be cleaned sufficiently with aqueous cleaners.
In most cases, however, at least some level of residual flux is left behind. These residues have the potential to corrode the solder joints, creating a concern regarding reliability. In the field of repair and rework fluxes, this potential to corrode solder joints is a particularly important one for two reasons. First, the time between repair/rework and cleaning is typically longer than the time between primary attachment and cleaning. In primary attachment, cleaning is usually performed within ten minutes of the attachment. On the other hand, several hours may follow between rework/repair and cleaning. Next, a low melt tin-lead-indium solder is utilized for pin-in-hole component rework. This solder has a greater propensity to corrode than the normal tin lead eutectic solder.
In water-soluble solder fluxes, the activator components promotes the corrosion of the solder. Typical activator are either hydrohalide derivatives such as dimethylamine hydrochloride or organic acids such as glycolic acid (hydroxyacetic acid). Presently water-soluble solder fluxes require thermal activation to produce the "active" state of the activator components.
It would, therefore, be desirable to have a water-soluble solder flux that does not cause corrosion of solder joints during rework or repair of circuit boards and that does not require thermal activation.